Ran the attached test case, clicking the 'Press Me' button. You get the following exception:

Exception in thread "AWT-EventQueue-0" java.lang.ArrayIndexOutOfBoundsException: 1
at javax.swing.SizeRequirements.expandedTile(SizeRequirements.java:355)
at javax.swing.SizeRequirements.calculateTiledPositions(SizeRequirements.java:299)
at javax.swing.BoxLayout.layoutContainer(BoxLayout.java:378)
at javax.swing.JToolBar$DefaultToolBarLayout.layoutContainer(JToolBar.java:762)
at java.awt.Container.layout(Container.java:1395)
at java.awt.Container.doLayout(Container.java:1384)
at java.awt.Container.validateTree(Container.java:1467)
at java.awt.Container.validateTree(Container.java:1474)
at java.awt.Container.validateTree(Container.java:1474)
at java.awt.Container.validateTree(Container.java:1474)
at java.awt.Container.validate(Container.java:1442)
at javax.swing.RepaintManager.validateInvalidComponents(RepaintManager.java:602)
at javax.swing.SystemEventQueueUtilities$ComponentWorkRequest.run(SystemEventQueueUtilities.java:127)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)

Also take a look at recent (01-Mar-2005) JDC comments to 4294758. I suspect those people are running into this bug.